His mom thinks his dad is cheating. How do you tell her that he’s not?

His mom thinks his dad is cheating. How do you tell her that he’s not?

“I’m 17 and I guess my mom found a bra in the living room that my girlfriend left when she came over. I pretended that I had never seen it, so later I heard my mom scream at dad and accuse him of cheating. I don’t know what to say because my mom is very traditional and thinks I’m still a virgin, but obviously I don’t want my dad taking the fall for it.
What’s the best thing I can do?”